feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/embedded-io/CHANGELOG.md

@@ -0,0 +1,54 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## 0.6.1 - 2023-10-22
+
+- Make `SliceWriteError` publicly available.
+
+## 0.6.0 - 2023-10-02
+
+- Prohibit `Write::write` implementations returning `Ok(0)` unless there is no
+  data to write; consequently remove `WriteAllError` and the `WriteAllError`
+  variant of `WriteFmtError`. Update the `&mut [u8]` impl to possibly return
+  a new `SliceWriteError` if the slice is full instead of `Ok(0)`.
+- Add `WriteZero` variant to `ErrorKind` for implementations that previously
+  may have returned `Ok(0)` to indicate no further data could be written.
+- `Write::write_all` now panics if the `write()` implementation returns `Ok(0)`.
+
+## 0.5.0 - 2023-08-06
+
+- Add `ReadReady`, `WriteReady` traits. They allow peeking whether the I/O handle is ready to read/write, so they allow using the traits in a non-blocking way.
+- Add variants to `ErrorKind` mirroring `std::io::ErrorKind`.
+- Add `From` impls to convert between `ErrorKind` and `std::io::ErrorKind`.
+- Moved `embedded_io::blocking` to the crate root.
+- Split async traits to the `embedded-io-async` crate.
+- Split trait adapters to the `embedded-io-adapters` crate.
+- Add `std::error` impls for `ReadExactError` & `WriteAllError`.
+- Rename trait `Io` to `ErrorType`, for consistency with `embedded-hal`.
+- Added optional `defmt` 0.3 support.
+
+## 0.4.0 - 2022-11-25
+
+- Switch all traits to use [`async_fn_in_trait`](https://blog.rust-lang.org/inside-rust/2022/11/17/async-fn-in-trait-nightly.html) (AFIT). Requires `nightly-2022-11-22` or newer.
+
+## 0.3.1 - 2022-10-26
+
+- Fix compilation on recent nightlies (#5)
+
+## 0.3.0 - 2022-05-19
+
+- `FromFutures` adapter now requires `futures` Cargo feature. (breaking change)
+- Add `FromTokio` adapter.
+- Add blanket impls for `&mut T`, `Box<T>`.
+- Add impl `Read`, `BufRead` for `&[u8]`
+- Add impl `Write` for `&mut [u8]`
+- Add impl `Write` for `Vec<u8>`
+- impl `std::error::Error` for `ReadExactError`, `WriteFmtError`.
+
+## 0.2.0 - 2022-05-07
+
+- First release